The present invention relates to thin-films on substrates. Specifically, the present invention relates to the use of Auger Electron Spectroscopy in the measurement of the thickness of an ultra-thin film on a substrate.
Ultra-thin films with thickness of less than five nanometers have found increasing application in the development of nano-technology. The exact thickness of such ultra-thin film is important for both the basic studies of multi-layer nano-structures and in the technical specifications of nano-device fabrication. For example, in the production of a magnetic head for a hard disc drive (HDD); an ultra-thin diamond-like-carbon (DLC) film is coated on the slider substrate, with an ultra-thin silicon (Si) film as the transition layer in between, to protect the writer/reader sensor. With the rapid increase of the HDD recording density to 80 G bits/inch2, the flying height of the slider over the disc has been reduced to about 10 nm. Correspondingly, the thickness of DLC and Si layer must be reduced to 1.5 to 2.5 nm, and the accurate measurement of the ultra-thin film thickness becomes a key factor in the advanced giant magnetic resistance (GMR) head research and manufacturing. Several nano-metrology techniques, which can meet the above demand to some extent, exist in the prior art.
As shown in FIG. 1a, an Atomic Force Microscope (AFM) may test the thickness of an ultra-thin film 102 coated on a substrate 104 by scanning with an AFM tip 106 the depth of a photoresist-created groove 108 on the film 102. AFM has a resolution of 10 nm on the X-axis and Y-axis and a resolution of 0.1 nm on the Z-axis. The disadvantage of AFM is that it requires elaborate sample preparation and can only measure the thickness of a mono-layer or the combined total thickness of a multi-layer film. Therefore, the usefulness of AFM is limited to calibration applications.
As shown in FIG. 1b, a Transmission Electron Microscope (TEM) is a superior technique to measure an ultra-thin diamond-like carbon (DLC) layer thickness. A TEM is powerful in nano-scale dimension measurement, being able to directly observe and measure the ultra-thin film thickness at the one-millionth resolution, or even higher magnification with spatial resolution of about 0.1 nm. However, this method requires complicated and time-consuming sample preparation by a Focus Ion Beam (FIB), to create a cross section and, then, measuring the thickness by TEM. Such a “destructive” measurement is undesirable in many cases and not suitable for routing quality monitoring of a nano-device production.
Electron Spectroscopy for Chemical Analysis (ESCA) is a third known method for nano-thin film thickness measurement. ESCA is a non-destructive method used to measure the diamond-like carbon thickness on a silicon chip with an area resolution of approximately 1 mm, and in some cases up to 10 μm. As shown in FIG. 1c, the sample has a substrate 110 composed of Al2O3TiC and is covered with a silicon layer 112 and a DLC layer 114. In this ESCA method, incident X-ray photon 116 causes the photoelectron emission at a specific angle (θ) from the surface of the DLC layer 114. Using the concentrations of silicon (Si %), aluminum (Al %), and carbon (C %) in the sample, calculated from the ESCA measured Si, AL, and C photoelectron signal intensities, the thickness of the DLC layer d 114 can be determined with the following equations:
      d    =          λ      ⁢                          ⁢      sin      ⁢                          ⁢              θ        ·                  ln          ⁡                      (                          1              +              aR                        )                                          a      =              A        ·                              BR            ′                                1            +                          BR              ′                                            ,                  R        ′            =                        Si          ⁢          %                          Al          ⁢          %                      ,          R      =                                    C            ⁢            %                    -                      J            *            Si            ⁢            %                                    Si          ⁢          %                                A      =                        I          Si          ∞                          I          C          ∞                      ,          B      =                        I          Al          ∞                          I          Si          ∞                    The variable a represents the intensity factor, the variables R and R′ represent modification factors, the variable λ represents the attenuation length of a Si photoelectron passing by carbon layers, and J represents the ratio of SiC in Si layer. Using the same inputs, the thickness of the silicon layer d′ 112 can be determined using the following equations:
            d      ′        =                  λ        ′            ⁢      sin      ⁢                          ⁢              θ        ·                  ln          ⁡                      (                          1              +              Br                        )                                    r    =                  Si        ⁢        %                    Al        ⁢        %            The variable r represents modification factors and the variable λ′ represents the attenuation length of the A1 photoelectron passing by the silicon layer. However, the area resolution of the ESCA method is normally in the millimeter range and about 10 μm at best. Therefore, this technique is not applicable to the thickness measurements of small area, such as down to the sub-micrometer, or even nanometer size, that is absolutely important for a submicro-device or nano-device research and manufacturing.
An Auger Electron Spectroscopy (AES) depth-profile method can be used to measure the relative thickness of an ultra-thin film. AES is an advanced solid surface analysis technique, based on the “Auger Effect”. The Auger process is initiated with the removal of a core electron by an energetic primary electron beam, creating an ion with an inner shell vacancy. In the relaxation of the exited ion, an electron from a higher energy level fills the inner shell vacancy with the simultaneous emission of another higher level electron, called an “Auger Electron”. The kinetic energy of the Auger electron is determined by the energy difference of the related 3 levels and is characteristic for the atom in which the Auger process occurs. An Auger electron spectrum plots the number of electrons detected as a function of electron kinetic energy. Elements are identified by the energy positions of the Auger peaks, while the concentration of an element is related to its Auger signal intensity. Furthermore, both theoretical and experimental research have shown that the mean free path of an Auger electron emission is less than 5 nm, meaning only a few to a dozen atomic layers can be detected by AES. AES is thus applied for the elementary composition analysis of a solid surface. AES and an Argon ion beam etching of the surface may be combined to perform a compositional depth profile analysis. Therefore, the thickness of a thin film can be obtained by taking account of the Argon ion sputtering time to remove the thin film layer, with the calibrated sputtering rate in hand. This method is also a destructive method and the accurate stuttering rate for a certain material is often difficult to obtain.
Lastly, an AES physical-mathematical model, for measuring the thickness of Cobalt thin film on a Nickel-Iron alloy substrate, has been reported. An example, as shown in FIG. 1d, includes a substrate 118 with a thin film 120 layer applied. The substrate may be a nickel-iron alloy, with the thin film layer 120 made of cobalt. The angle 122 of an emission Auger electron 124 from normal is 43°. An AES physical-mathematics model method may be used to determine the thickness 126 of the thin film layer 120. The mathematics model is as follows:
      t    =                  λ        Ni        Co            ⁢      cos      ⁢                          ⁢      θ      ⁢                          ⁢      ln      ⁢                          ⁢              (                                            I              Co                                      a              *                              I                NiFe                                              +          1                )                  a    =                  I        Co        ∞            /              I        NiFe        ∞            The variable λNiCo represents the attenuation length of Auger electron of Ni element in substrate layer 118 through Co layer 120. This method applies only to a monolayer Cobalt film on a nickel-iron alloy substrate.
What is needed is a way to measure ultra-thin film (≦5 nm) layers of either single or double layers in an area of about 100 nm2. For example, in the manufacturing of magnetic recording head, a key component device for computer Hard Disk Drive (HDD), the slider surface is coated with an ultra-thin film of diamond like carbon (DLC), with Si as the transition layer between DLC and AL2O3—TiC substrate. The flying height of a slider over a disk is currently down to about 10 nm. To control the thickness of the DLC and Si layer to between 2.5 nm and 1.5 nm becomes essential for the magnetic head production. Meanwhile, the key components of the slider, the writer and reader sensor are of a submicrometer or nanometer size, making the nano-metrology of the precise thickness measurement of the double layer ultra-thin film is vitally important for high-tech production. In addition, the measurement is supposed to be done without the need for a complicated sample preparation, or non-destructive and efficient enough to satisfy industrial testing.